For some time now I have been getting flat batteries - a condition which seems to be accelerating recently. To cut a long story short I have checked out all the obvious things and after many hours of sleuthing I now have narrowed down the potential culprit.
My multimeter tester shows an ongoing 230/50 MilliAmp draw and this only quits with the removal of both fuses Fuses #8 and #9 situated in the trunk fuse box. My reference guide for fuse function shows many items linked to these fuses but only in one instance do BOTH refer to the same item and that is the alarm system. I remember years ago the alarm siren running but I have never heard it since - nor can I make it actuate. I am thinking that maybe the alarm siren IS running all the time but it cannot be heard? Is this possible?
My quest now is to find the alarm siren and disconnect it and see if this will have any effect. There is one problem here however! I don't know where the siren is located.....I read on the forums "behind the fog lamp" "behind the wheel arch housing" "on the drivers side" "on the passengers side".....I have looked behind both fog lamps and I don't see anything there.....Does someone know where it is located? That will be a start to find it and do a disconnect.
As usual any help greatly appreciated.

Interesting post here...because I also have never heard my own ATA horn sound off, even though I have done what should have triggered it. I suspect that it was disconnected by the PO, and as I do not care for these audible alarms (when they misbehave) I have not even looked for a solution.

As for this problem above, the alarm horn is mounted out of sight, behind the right fender inner liner, as shown on the WIS drawing below. (only remove the front section for access) The horn has a rechargeable battery incorporated, which may have died.

Referring to the 2 fuses, I think that you should find out which one is the problem. It is unlikely to be both:

Hey! That drain might be normal alarm electronics current drain especially if your car has Keyless-GO, like my car. If it sits more than 30 days, the starting is hard, so I start it every 2 weeks when I am driving the other cars.

On my wife's Prius, I turned off the Keyless-GO for that exact reason... there is a Power switch under the steering column to disable it. Infact, the Owner's manual tells you to do just that if away on a long vacation.

I don't think Keyless-GO can be turned off by the owner, unless Keyhole can find something in the WIS about turning it off?

Keyless Go is a nice feature but the Prius has a tinnie weinnie 12V NiMH battery which keeps up it's charge only by daily driving... so I pushed the OFF switch to disable the keyless-GO feature.

Problem solved!
It took a while but patience paid off.....
After going through the procedure of checking each fuse for battery drain it showed that when fuse #8 was removed the drain dropped from 130ma down to 20ma. This took quite a while though - probably 5/10 minutes for the system to stabilise but it was evident that something positive was happening as there was an immediate reduction of drain after the fuse was removed. The fuse info sheet indicated that #8 was allocated to "convenience locking, anti theft alarm and remote trunk release". It occurred to me that I had not heard the alarm siren for a year or two and frankly hadn't thought much about it - UNTIL NOW!
So the next step was to find the alarm, isolate it and check the drain. Easier said than done.....lots of false starts here. After looking behind the fog lamps etc. it was found exactly where Keyhole described "behind the REAR wheel arch panel on the RHS, which in my case being in British Columbia was on the passenger side of the car.
I unbolted the alarm, disconnected it and Shazam! draw dropped to 20ma after about 5 minutes.....
The alarm is supposedly a sealed unit and I needed a hacksaw to cut the seam through to split the unit in half. There was the problem staring me in the face...a totally corroded the circuit board - so much for being sealed...
I have spent a lot of time in coastal Mexico so I imagine it was the salt air that somehow got into the unit and hastened the corrosion. No I will not replace it...I just taped up the connections and put the panel back. So ends dozens of hours of poking, probing, rolling around on the garage floor, replacing fried multi testers etc. etc. Rewarding though - the local Stealership wanted $113.00 per hour and said there were no estimates what it might cost - they had just had a similar problem (that was a defective locking system) and they billed 10 hours for that!'
The old alarm module? Well it might wind up on the Christmas tree!